1. Field of the Invention
The present invention relates generally to the field of optical fiber cables and more particularly to a method of color coding the surface of an optical fiber with an ultraviolet light curable color coding ink.
2. Description of the Prior Art
The demands for optical fiber cable are ever increasing thereby necessitating more efficient and better optical fiber cable manufacturing processes for all optical fiber cable structure families. Optical fiber cable structure families which are currently being used are loose tube, slotted core, single tube and tight tube. In a loose tube structure, several gel filled buffer tubes containing optical fibers are stranded around a central strength member. The slotted core structure has optical fibers precisely placed in gel filled channels or slots. The channels are symmetrical and form a helical path along the longitudinal axis of the cable. A strength member is located in center of the slotted core cable structure. In the single tube structure, all of the optical fibers are in a single, centrally located, gel filled buffer tube. Finally, in the tight buffer tube construction, protective layers are applied in direct contact with each optical fiber so there is no fiber overlength. All the above cable structures may incorporate various sheath constructions (armored, non-armored, high strength, self-supporting, flame retardant, etc.) depending on the application and environment.
Similar to individual metallic conductors in a conventional metallic communication transmission cable, each of the optical fibers in an optical fiber cable are are color coded so as to identify each optical fiber. Typically, such color codings are achieved by applying either a polymeric color coding ink or an ultraviolet curable color coding ink to the surface of the optical fiber prior to manufacture of a finished optical fiber cable.
Polymeric color coding inks typically comprise a solid polymeric resin and pigment dissolved in a solvent. Such inks are applied by passing the optical fiber to be color coded through a rigid die applicator which is wetted with the polymeric resin and pigment solvent solution so as to contact and wet the surface of the optical fiber. After the fiber surface is wetted with the polymeric resin and pigment solvent solution, the fiber passes through a thermal oven to evaporate the solvent from the surface of the optical fiber. After evaporation of the solvent, only the polymeric resin and pigment remains on the fiber surface. Unfortunately, the solid polymeric resin pigments left on the fiber surface tend to have some reactivity with the other components of the cable such as the water blocking gel filling compounds, e.g. polyisobutylene. Also, the cables are sometime subjected to high processing temperatures and the polymeric resin and pigments tend to have thermoplastic properties which makes them flow or remelt when sufficient heat is applied to the polymeric resin and pigment on the optical fiber surface.
Ultraviolet light curable color coding inks can be used to overcome the reactivity and thermoplastic property problems associated with using solid polymeric resin and pigments. Ultraviolet curable color coding inks are typically applied as 100 percent ink solids. Such inks are applied by passing the optical fiber to be color coded through at least one rigid die applicator which is wetted with the ultraviolet ink. The applicator die contacts and wets the surface of the optical fiber with the ultraviolet ink. The wetted fiber is then passed through an ultraviolet light radiator to initiate a cross-link curing of the ink on the fiber surface. Although the ultraviolet inks add the advantage of being chemically inert after curing, unfortunately, when coating fibers with these ultraviolet inks, breakages of the fiber commonly occur. Such breakages are caused by a pinching of the fiber by the applicator dies. The pinching is a result of the need to place the applicator dies in very close proximity to the fiber surface in order to place a thin uniform coating on the surface. These breakages result in loss productivity and increase costs.
It is also desirable to have thin coatings on the optical fibers because the ultraviolet curable inks generally have a different thermal expansion coefficient than glass of the optical fibers. The difference in thermal expansion coefficients can cause the fibers to undergo microbending where a cable is exposed to extreme temperature changes. Thus, a thin coating helps to minimize this phenomena by keeping the percentage of material with a different coefficient of thermal expansion in contact with the optical fibers to a minimum.
Therefore, it would be desirable to develop a solution to the shortcomings found in the prior art.